1. Field of the Invention
This invention concerns a ferritic stainless steel plate and a manufacturing method and, more in particular, it relates to a ferritic stainless steel plate which, throughout this specification and claims also includes steel strip, the plate having excellent ridging resistance and formability such as press workability and bendability.
2. Description of the Related Art
Ferritic stainless steels have been utilized in various fields such as kitchen utensils or automobile parts since they resist formation of stress corrosion cracks, and are inexpensive, and have improved deep drawing properties and ridging resistance.
As the field of use for the ferritic stainless steels has been extended, more stringent standards have been demanded also for other types of formability characteristics, such as bulging properties or bendability, in addition to deep drawing properties and ridging resistance. The bulging property of the plate is a measure of how much a central portion of the plate can be bulged without breakage when it is bulged by pressing with the plate ends constrained. This is indicated by the bulging height, which is distinguished from the deep drawing property (evaluated as the xe2x80x9cr valuexe2x80x9d) by pressing without constraining the plate ends.
For improving the deep drawing properties and ridging resistance of the ferritic stainless steels, a technique for controlling colonies in the steel plates has been proposed recently.
According to the studies so far on colonies which are defined as groups of crystal grains having identical orientation, it has been considered most effective for the improvement of ridging resistance to make the colony smaller. For example, Japanese Patent Laid-Open NO. 330887/1998 discloses a method of improving ridging resistance by defining the length of the colony in the direction of the plate thickness within an RD (rolling direction as shown in FIG. 6, hereinafter simply referred to as the RD) plane to 30% or less of the plate thickness, thereby reducing the size of the colony in the direction of the plate thickness, and improving the deep drawing properties by defining the volumetric ratio of a {111} orientation colony to 15% or more, as shown in FIG. 6.
On the other hand, there has been an attempt of utilize specified colonies. For example, Japanese Patent Laid-Open No. 263900/1997 discloses the technique of defining the size of the {111} orientation colony in the direction of the plate width to 100-1000 xcexcm, thereby improving the ridging resistance of the plate and increasing the ratio of the {111} orientation colony in the direction of the plate width to improve the deep drawing property (r value).
In any of the methods described above, it is intended to improve the deep drawing property (r value) by causing a great amount of the {111} orientation colony to exist, and to improve the ridging resistance of the plate by making the size of the {111} orientation colony smaller.
However, although the deep drawing property and the ridging resistance can be improved by the techniques described above, it is difficult to remarkably improve also the bulging property of the plate. Japanese Patent Laid-Open No. 310122/1995 discloses a technique of improving ridging resistance together with pressing workability. This intends to improve the deep drawing property (r value), the ridging resistance and the bulging property together by controlling the temperature for at rough rolling (1000 to 1150xc2x0 C.), friction coefficient (0.3 or less), rolling reduction (40-75%) and strain rate (7-100 1/s) thereby promoting recrystallization at the center of the plate thickness. However, even this technique can not effectively cope with the demand for large bulging capability in recent years.
On the other hand, since cracks have sometimes occurred upon severe bending of stainless steel plates, the bending resistance has also become one of the important characteristics required. Cracks upon bending have been discussed mainly in view of non-metal inclusion in the steels. Particularly it has been known that xe2x80x9cA type inclusionsxe2x80x9d (No. 3132 defined by JIS(Japanese Industrial Standard)G0202) extended in the rolling direction, located just beneath the surface of the steelplates, give undesired effects (xe2x80x9cIron and Steelxe2x80x9d by Otake, et al, 46 (1960), p. 1273) For instance, Japanese Patent Laid-Open No. 239600/1993 discloses a method of improving bendability by replacing A type inclusions suffering from work-induced plastic deformation with xe2x80x9cC type inclusionsxe2x80x9d (No.3134 defined by JIS G0202) such as granular oxides dispersed irregularly in the steels with no plastic deformation.
Further, Japanese Patent Laid-Open No. 306435/1993 discloses a method of attaining improvement of the bendability characteristics by making the purity higher, such as Fe+Crxe2x89xa799.98 wt % in Fexe2x80x94Cr alloys.
Further, Japanese Patent Laid-Open. No. 104818/1974 discloses a technique of improving bendability by controlling chemical compositions as Mn/Sixe2x89xa71.4 and decreasing MnO.SiO2 type inclusions.
However, since each of the techniques described above is a method of controlling the ingredients in the steels, it involves a problem of increasing production cost and production and, thus, resulting in reduction of productivity.
In view of the above, it is an object of this invention to overcome the problems in the prior art described above, and to create a ferritic stainless steel plate having excellent ridging resistance and formability (such as deep drawing, bulging and bendability), as well to provide a novel manufacturing method.
This invention further has, as an object, to provide a ferritic stainless steel plate having excellent ridging resistance and formability, as well as a manufacturing method, with no particular requirement of special chemical compositions such as reduced content of C or N, addition of Ti or Nb, high purification or control of the Mn/Si rates.
We have carefully studied the relationship between the ridging and the crystal orientation distribution in the direction of the plate thickness, for attaining the foregoing purpose. As a result, we have discovered a new way of improving ridging resistance and formability (such as the deep drawing, bulging and bendability) of general purpose ferritic stainless steel plates typically represented by SUS430 and the like. We have discovered that it is important to positively utilize a {111} orientation colony and, particularly, that it is extremely effective to control the colony in a specified position within the transverse direction (TD) plane of the plate, hereinafter simply referred to as the TD plane. It is important specifically, to distribute more {111} orientation colonies in the two regions which comprise xe2x85x9 to xe2x85x9c and ⅝ to xe2x85x9e of the plate thickness, in which columnar crystals are formed within the cross section in the direction of the plate thickness. Further, it has also been found that plate bendability is further improved by controlling the mean crystal grain size of the steel within a predetermined range.
(1) The ferritic stainless steel plate of this invention has the following characteristics:
The area ratio of {111} orientation colonies, defined as below measured, in the cross section in the direction of the plate thickness cut into a rolling direction, is defined to be about 30% or more in the regions extending from xe2x85x9 to xe2x85x9c, and the regions extending from ⅝ to xe2x85x9e of the plate thickness within the cross section, in the direction of the plate thickness:
The {111} orientation colony is an assembly of adjacent crystals in which the angle xcex1 of the  less than 111 greater than  direction vector of each crystal relative to the orientation vector vertical to the rolling surface, is within 15xc2x0. That is shown as the orientation of the normal direction in FIG. 6, hereinafter referred to as the xe2x80x9cNDxe2x80x9d orientation.
The rolling surface indicates the surface of the rolling material. Referring to FIG. 6, this is a surface in parallel with the ND plane, which indicates the top surface or bottom surface of the rolling material.
(2) A ferritic stainless steel plate having excellent ridging resistance and formability as defined in (1) above, wherein the mean crystal grain size is from about 3 to 100 xcexcm, preferably, about 3 to 60 xcexcm.
(3) A method of manufacturing a ferritic stainless steel plate having excellent ridging resistance and formability by rough rolling and finish rolling slabs in hot rolling, applying annealing and cold rolling to the hot rolled plates and then applying finish annealing, wherein the rough rolling is conducted at a rolling reduction in at least one pass in the rough rolling step of the hot rolling of about 30% or more, and at, a temperature difference, between the center of the plate thickness and the plate surface, of about 200xc2x0 C. or lower in the pass where the rolling reduction is maximum.